An InGaAsP system embedded type semiconductor laser is generally used as a light source in light communication for long distance and large capacity transmission. FIG. 6 shows a cross-section of a prior art embedded type semiconductor laser which is called a Buried Crescent (hereinafter referred to as BC) laser. The BC laser is described in detail in the IEEE Journal of Light-Wave Technology, Volume LT-3, page 978, 1985.
In FIG. 6, the reference numeral 1 designates a substrate comprising p type InP. The numeral 2 designates a first current blocking layer comprising n type InP. The numeral 3 designates a second current blocking layer comprising p type InP. The numeral 4 designates a first cladding layer comprising p type InP. The numeral 5 designates an active layer. The numeral 6 designates a second cladding layer comprising n type InP. The numeral 7 designates a p type electrode. The numeral 8 designates an n type electrode. The numeral 9 designates a pn junction region.
The semiconductor laser of FIG. 6 is produced in a sequence of steps.
A first current blocking layer 2 and a second current blocking layer 3 are successively grown on the substrate 1. Next, a groove having an arrowhead configuration is produced in the (011) direction, usually by photolithography and chemical etching. Hydrochloric acid is used for the etching. The width of the groove is established at a value less than 2 micrometers so that the transverse oscillation may be a single fundamental mode.
Next, the first cladding layer 4 and the active layer 5 are successively grown on the substrate 1 in the groove, and thereafter a second cladding layer 6 is grown on the active layer 5 and the second current blocking layer 3.
After the second crystal growth is concluded, a p electrode 7, such as AuZn or Au, is deposited on the substrate 1 opposite blocking layer 2, and an n electrode 8, such as AuGe or Au, is deposited on the second cladding layer 6 opposite blocking layer 3 to complete a BC laser as shown in FIG. 6.
When a bias voltage is applied to the electrodes so that the n electrode 8 is negative relative to electrode 7, the pn junction produced by the active layer 5 (which is normally p type) and the second cladding layer 6 becomes forward biased. The pn junction region 9 produced by the first current blocking layer 2 and the second current blocking layer 3 becomes reverse biased. Accordingly, the path of the curent flowing through the chip is restricted to the substrate 1, the first cladding layer 4, the active layer 5, and the second cladding layer 6. Current does not flow through any region other than active layer 5. As a result, almost all the current inside the chip is concentrated on the active layer 5, thereby contributing to the laser oscillation. Thus, a low threshold current, such as lower than 20 mA, is realized in the BC laser.
In the BC laser described above, reverse bias junctions are provided on both sides of the active layer 5 in order to concentrate current flow in a so-called embedded type low threshold current semiconductor laser. That construction has a large capacitive component from the reverse bias junctions. Since the RC time constant of the laser is large because this capacitive component functions as a parasitic capacitance, the frequency response of a semiconductor laser employing this structure has been restricted to a value lower than a few hundred MHz.